Application of Unit Protection Schemes for Auto-Transformers. Zoran Gaji ć ABB AB Vasteras, Sweden. Authors: Z. Gaji ć, ABB Sweden S. Holst, ABB Sweden. Auto-Transformer. An auto-transformer is a power transformer in which at least two windings have a common part

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Application of Unit Protection Schemes for Auto-Transformers

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Auto-Transformer

An auto-transformer is a power transformer in which at least two windings have a common part

Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer)

In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower the zero sequence impedance for five-limb core constructions or for auto-transformers built from three single phase units

Example Auto-Transformer

Auto-Transformer has Dual Rating

It can be shown that power is transferred in two different ways through an auto-transformer.

One part of the power is transferred by galvanic connection and the other part is transferred via magnetic circuit (i.e. transformer action)

Auto-transformer is cheaper that corresponding two/three winding power transformer design

Possible problem with short circuit current withstand

One three-phase unit

Typically has five-limb core

Three single-phase units connected to form three-phase group

Auto-transformer Construction

Possible CT Locations for Auto-Transformer

Based on autotransformer ampere-turn balance 87T

Based on the first Kirchhoff’s law between galvanically interconnected parts 87B

Possible Differential Protection Principles

Restricted Earth-Fault

Zero-sequence current based

Dedicated delta winding unit protections

Special Unit Protections

87T Differential protection using CT1 and CT2

SBase=Throughput Power(400MVA)

Tertiary delta winding can not be loaded

Mandatory zero sequence current reduction

87T Differential protection using CT1 and CT2

87T Differential protection using CT1, CT2 and CT3

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

Mandatory zero sequence current reduction

87T Differential protection using CT1, CT2 and CT3

87T Differential protection using CT1, CT2 and CT7

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

CT location within delta winding requires “special attention”

Zero sequence current reduction not required if it is a five-limb or single-phase construction

87T Differential protection using CT1, CT2 and CT7

* Influenced by CT location within tertiary delta winding

87T Differential protection using CT1, CT5 and CT7

SBase=Magnetic Power(169MVA)

Tertiary delta winding can be loaded

CT location at neutral point and within delta winding requires “special attention”

Zero sequence current reduction not required if it is a five-limb or single-phase construction

MV (i.e. 220kV) bushings not protected!

87T Differential protection using CT1, CT5 and CT7

Influenced by CT5 location in neutral point

* Influenced by CT7 location within tertiary delta winding

87T Differential protection using CT1, CT2, CT7 and CT8

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

Double CT location within delta winding requires “special attention”

Relay with four restraint inputs required

Mandatory zero sequence current reduction

87T Differential protection using CT1, CT2, CT7 and CT8

* Influenced by double CT location within tertiary delta winding

87T Differential protection using CT1, CT5, CT7 and CT8

SBase=Magnetic Power(169MVA)

Tertiary delta winding can be loaded

CT location at neutral point and within delta winding requires “special attention”

Relay with four restraint inputs required

Zero sequence current reduction not required if it is a five-limb or single-phase construction

MV (i.e. 220kV) bushings not protected!

87T Differential protection using CT1, CT5, CT7 and CT8

Influenced by CT5 location in neutral point

* Influenced by CT7, CT8 location within tertiary delta winding

87B Differential protection using CT1, CT2 and CT6

SBase=Throughput Power(400MVA)

Zero sequence current reduction not required

Tertiary delta winding can be loaded

Not sensitive for winding turn to turn faults

87B Differential protection using CT1, CT2 and CT6

REF protection using CT1, CT2 and CT4

Base quantity is current (either CT2 or W2 rating)

Tertiary delta winding can be loaded but it is not protected

Operates only for phase to ground faults

REF protection using CT1, CT2 and CT4

Dedicated unit scheme for tertiary winding

Use simple I> relay to provide only earth-fault protection for tertiary delta winding

Field Recording

CT1

CT5

CT2

CT4 & CT7

External L2-Gnd fault which before clearing evolved into a L2-L3-Gnd fault

CT1, CT2, CT4 and CT7 currents recorded

CT5 current calculated

Delta winding not loaded, thus CT7 currents are identical in all three phases

Calculated Diff Currents from Field Recording

Calculated REF Diff Current from Field Recording

Conclusion

The following data are crucial for proper application of the selected differential protection scheme for Auto-transformer:

Which base quantities (i.e. power, no load voltage and current) shall be used